1. Field of the Invention
The present invention generally relates to heat exchangers, and in particular, to steam generators used in nuclear reactors.
2. Description of the Prior Art
A heat exchanger is an apparatus used to transfer heat from a fluid flowing on one side of a barrier to another fluid or fluids flowing on the other side of a barrier. One of these fluids is designated the heating fluid and the other, the heated fluid.
In nuclear power plants the heat exchanger used to make steam is commonly called a steam generator. The nuclear reactor generates heat which is transferred to the steam generator by a flow of primary coolant. The primary coolant is the heating fluid and causes the water in the steam generator to form steam. The steam is typically used to drive turbines and generate electrical power.
Recently, research has been conducted in developing a fast breeder reactor for commercial electric power stations. The present state of this research is outlined in the article entitled "Superphenix: A Full-Scale Breeder Reactor," Scientific American, March 1977, Vol. 236, No. 3, pp. 26-35.
The primary coolant in a typical fast breeder reactor is sodium. At atmospheric pressure sodium is liquid at 90.degree. Celsius and boils at 882.degree. C. A fast breeder reactor generates heat which is transferred to an intermediate heat exchanger through a primary sodium flow circuit. The intermediate heat exchanger is in turn cooled by a secondary sodium circuit which transfers the heat from the fast breeder reactor to a steam generator to form steam. In normal operation a typical fast breeder reactor has a temperature of 395.degree. C. at the inlet of the reactor and a temperature of 545.degree. C. at the outlet.
Heretofore, one problem with operating at these high primary coolant temperatures has been avoiding severe thermal transients in the water-steam circuit of the steam generator. A fast breeder reactor has the potential for developing high heat flux, and if this heat flux is not carefully regulated, it can quickly overcome the heat transfer capability of the steam generator. Most steam generators today are designed to avoid departing from nucleate boiling (DNB) in the water tubes of the steam generator. When a departure from nucleate boiling occurs, the bulk temperature of the water exceeds its saturation temperature. Bubbles begin to form throughout the water in the steam generator and the rate of heat transfer from the primary coolant markedly decreases. The bubbles tend to block the flow of water through the water tubes and a blanket of steam envelopes the tubes. This phenomenon, called bulk boiling, substantially reduces the transfer of heat from the tube wall to the water in the steam generator. The wall temperature of the water tubes rapidly climbs and the steam generator experiences a rapid temperature excursion.
In a fast breeder reactor departure from nucleate boiling is an especially critical design problem because the temperature of the primary coolant is high. In turn, the average temperature of the water in the steam generator is closer to its saturation temperature. Thus, the thermal margin to DNB is smaller than in other reactor power plants.
Prior approaches to the problem of insuring that DNB does not occur have suggested positioning protective tubes around the water tubes. Another concept is the placement of internal turbulators within the water tubes to break up the blanket of steam that accompanies bulk boiling. In addition, the use of smaller diameter water tubes has been suggested along with increasing the rate of flow of water through the water tubes.
In general, these prior approaches all significantly increase the complexity of the design of a steam generator along with the cost of its fabrication and operation. In addition, these prior approaches increase the risk of malfunction and the possibility of leakage between the sodium and the water. Lastly, and more importantly, these prior designs do not totally insure that departure from nucleate boiling will not occur in the water tubes of the steam generator.